Chrysler Hemi engine

Early Hemi in a 1957 Chrysler 300C
The Chrysler Marine Hemis were popular in wooden boats during the 1950s and 60s

The Chrysler Hemi engines, known by the trademark Hemi, are a series of I6 and V8 gasoline engines built by Chrysler with hemispherical combustion chambers. Three different types of Hemi engines have been built by Chrysler for automobiles: the first (known as the Chrysler FirePower engine) from 1951 to 1958,[1] the second from 1964 to 1971, and the third beginning in 2003. Although Chrysler is most identified with the use of "Hemi" as a marketing term, many other auto manufacturers have incorporated similar designs.

During the 1970s and 1980s, Chrysler also used the Hemi name for their Australian-made Hemi-6 Engine and applied it to the 4-cylinder Mitsubishi 2.6L engine installed in various North American market vehicles.

Concept

A hemispherical cylinder head ("hemi-head") gives an efficient combustion chamber with an excellent surface-to-volume ratio, with minimal heat loss to the head, and allows for two large valves. However, a hemi-head allows no more than two valves per cylinder, and these large valves are necessarily heavier than in a multi-valve engine. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Since the combustion chamber is a partial hemisphere, a flat-topped piston would yield too low a compression ratio unless a very long stroke is used, so to attain desired compression ratio the piston crown is domed to protrude into the head at top dead center, resulting in a combustion chamber in the shape of the thick peel of half an orange.

The hemi-head design places the spark plug at or near the center of the chamber to promote a strong flame front. However, if the hemi-head hemisphere is of equal diameter to the piston, there is minimal squish for proper turbulence to mix fuel and air thoroughly. Thus, hemi-heads, because of their lack of squish, are more sensitive to fuel octane rating; a given compression ratio will require a higher octane rating to avoid pre-detonation in a hemi engine than in some conventional engine designs such as the wedge and bathtub.

The hemi head always has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and single overhead cam engines (dual overhead cam engines may not have rocker arms). This adds to the overall width of the engine, limiting the vehicles in which it can be installed.

Significant challenges in the commercialization of engine designs utilizing hemispherical chambers revolved around the valve actuation, specifically how to make it effective, efficient, and reliable at an acceptable cost.[citation needed] This complexity was referenced early in Chrysler's development of their 1950s hemi engine: the head was referred to in company advertising as the Double Rocker Shaft head.[2]

WWII

Chrysler developed their first experimental hemi engine for the Republic P-47 Thunderbolt fighter aircraft. The XIV-2220 engine was an inverted V16 rated at 2,500 hp (1,860 kW). The P-47 was already in production with a Pratt & Whitney radial engine when the XIV-2220 flew successfully in trials in 1945 as a possible upgrade, but the war was winding down and it did not go into production. However, the exercise gave Chrysler engineers valuable research and development experience with two-valve hemi combustion chamber dynamics and parameters.

In addition to the aircraft engine, Chrysler and Continental worked together to develop the air-cooled AV-1790-5B V12 Hemi engine used in the M47 Patton tank.

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